Spatially, charge separation encompasses bulk diffusion to the semiconductor surface and interfacial transfer between different components in composites^[12]. In a pristine photocatalyst, the bulk carrier diffusion length critically dictates the efficiency of charge separation and subsequent participation in CO₂ reduction^[13]. If the bulk diffusion path is too long, photogenerated electrons will suffer from severe bulk recombination before reaching the active surface sites, rendering even strong light absorption ineffective. Conversely, minimizing the diffusion distance facilitates rapid carrier migration to the active sites. Consequently, engineering low-dimensional nanostructures or hierarchical porous architectures is a highly effective strategy for optimizing bulk charge separation.

However, the performance of bare photocatalysts remains constrained by rapid bulk charge recombination. To mitigate this, composite photocatalysts are constructed by integrating two semiconductors with appropriately matched energy bands^[7]. Based on their relative band alignments, heterojunctions are generally classified into three configurations [Figure 2A]: type-I (straddling gap), type-II (staggered gap), and type-III (broken gap)^[14]. In a type-I alignment, both electrons and holes transfer to the narrower-bandgap semiconductor, failing to achieve spatial charge separation. Meanwhile, for a type-III alignment, the extreme band offset renders continuous charge transfer energetically prohibitive. In a type-II heterojunction, electrons migrate to the lower conduction band while holes transfer to the higher valence band. This staggered transfer effectively separates the electron-hole pairs spatially, thereby enhancing photocatalytic performance^[2]. However, because electrons accumulate at a less negative conduction band potential, the system’s overall reductive driving force is compromised, which significantly limits multi-electron CO₂ reduction kinetics. Alternatively, staggered band structures can facilitate a completely different charge transfer mechanism known as a Z-scheme system, where the charge flow resembles the letter “Z” [Figure 2B]. Depending on the charge-mediating mechanism, Z-scheme systems are classified into traditional (using a redox mediator), all-solid-state (using a solid conductor), and direct Z-scheme (without an intermediate medium) heterojunctions. Compared with traditional Z-scheme and all-solid-state Z-scheme, the direct Z-scheme heterojunction can avoid the selection and shielding effects of electron media and reverse photochemical reactions^[14]. In this unique pathway, the photogenerated electrons with weaker reductive capability preferentially recombine with holes possessing weaker oxidative capability. Consequently, the strongly reductive electrons and strongly oxidative holes are preserved in their respective conduction and valence bands. Thus, the direct Z-scheme configuration successfully circumvents the redox potential loss inherent to type-II systems. Therefore, constructing appropriate heterojunctions is a critical strategy for enhancing interfacial and spatial charge separation. XPS is an important means for determining the chemical composition and electronic states of materials^[15].

Photogenerated charge separation at the interface of composite materials is a critical determinant of both the efficiency and selectivity of CO₂ reduction. A prominent example is the charge transfer mechanism in S-scheme heterojunctions, a concept initially introduced by Xu et al. in 2019^[15]. An S-scheme system consists of a reducing photocatalyst (RP) and an oxidizing photocatalyst (OP). Upon contact, the difference in their Fermi levels drives spontaneous electron transfer, establishing an internal electric field and inducing corresponding band bending at the interface. Under illumination, this built-in field, synergized with band bending, drives a macroscopic “step-like” interfacial charge transfer pathway [Figure 3A]. This mechanism selectively facilitates the recombination of the less reductive electrons in the OP and the less oxidative holes in the RP. Consequently, the strongly reductive electrons and strongly oxidative holes are preserved in their respective conduction and valence bands. Thus, interfacial charge separation in S-scheme systems is highly directional, achieving efficient carrier separation while maximizing the overall redox potential.

Furthermore, even if photogenerated carriers successfully migrate from the bulk to the semiconductor surface, they remain highly susceptible to rapid recombination if not properly extracted. Therefore, electrons must be further directed across the solid-liquid interface. To achieve this, metallic materials are typically deposited onto the semiconductor surface to form a Schottky junction [Figure 3B]^[16]. Because the metal generally possesses a larger work function, electrons at the semiconductor surface are preferentially injected into the metal, which acts as an effective electron trap. Ultimately, these electrons are successfully funneled to the solid-liquid interface, where they participate in the CO₂ reduction reaction on the catalyst surface. The formation of a Schottky barrier subsequently prevents electron backflow, ensuring the strict directionality of surface charge transfer.

In photocatalytic CO₂ reduction, charge separation efficiency is jointly governed by thermodynamic driving forces and kinetic competition^[17]. Thermodynamically, this depends on the band alignment between the semiconductor and the CO₂ reduction potentials, which must provide sufficient driving force for the redox reactions while minimizing excessive energy loss^[1]. The kinetic competition is primarily determined by the relative charge transfer rate (including interfacial charge injection and transport) and the charge recombination rate (non-radiative recombination in the bulk, surface, and interface)^[18]. When the charge transfer rate is significantly higher than the recombination rate, the charge separation efficiency is effectively enhanced. The charge separation efficiency can be expressed as^[18]: $$ \eta_{\mathrm{sep}}=\frac{\mathrm{K}_{\mathrm{ct}}}{\mathrm{~K}_{\mathrm{ct}}+\mathrm{K}_{\mathrm{rec}}} $$, where η_sep represents the charge separation efficiency, defined as the fraction of photogenerated electron-hole pairs that are successfully separated and subsequently participate in catalytic reactions. A higher η indicates a greater population of charge carriers effectively involved in catalysis, thereby corresponding to enhanced photocatalytic performance. This parameter fundamentally reflects the dynamic competition between charge transfer and recombination processes. The parameter K_ct denotes the charge transfer rate constant, which describes the kinetics of photogenerated carrier migration from the semiconductor bulk or interface to active reaction sites. This encompasses processes such as bulk-to-surface diffusion, carrier injection across semiconductor/cocatalyst interfaces, and electron transfer to adsorbed CO₂ molecules. A larger K_ct implies more efficient carrier delivery to catalytic sites, thereby facilitating reaction kinetics. In contrast, K_rec represents the charge recombination rate constant, describing the rate at which electrons and holes recombine, including bulk recombination, surface recombination, and interfacial recombination. An increased Krec indicates more severe carrier loss, which is detrimental to photocatalytic efficiency. When K_ct is much higher than K_rec, the photogenerated charges maintain an effective lifetime in a scale of nanosecond to microsecond. This prolonged lifetime provides a sustained electron supply for the continuous generation of key reactive intermediates (e.g., *CO₂ and *COOH). Furthermore, localized internal electric fields, Fermi level alignments, and adsorption-induced dipoles can accelerate interfacial charge injection. These factors enable the rapid transfer of electrons to metal, single-atom, or carbon-based active sites, while holes are concurrently scavenged by strong oxidation centers^[17]. This synergistic effect intrinsically suppresses recombination and drastically enhances the overall charge separation efficiency.

The Z-scheme heterojunction is derived from the “Z-scheme” electron transfer mechanism of natural photosynthesis^[36]. Its core advantage lies in achieving efficient separation of photogenerated electrons and holes while retaining high-energy electrons with strong reducing ability and high-energy holes with strong oxidizing ability to the greatest extent^[37]. This not only provides sufficient high-energy electron donors for the CO₂ photoreduction reaction but also lays a thermodynamic foundation for multi-step proton-coupled electron transfer (PCET) processes, making it an important design direction for catalytic systems to enhance the performance of CO₂ photoreduction.

Fundamentally, the direct Z-scheme heterojunction is an effective charge-transfer regulation strategy^[38], and its fundamental charge transfer mechanism is illustrated in Figure 5A. A typical Z-scheme system couples a semiconductor possessing strong oxidation capabilities (analogous to Photosystem II) with another possessing strong reduction capabilities (analogous to Photosystem I). After the two come into contact, the low-energy electrons in the conduction band of PS I and the low-energy holes in the valence band of PS II will preferentially recombine and be consumed at the interface; ultimately the high-energy electrons with strong reduction ability on the conduction band of PS II (directly used for the activation and reduction of CO₂), and the high-energy holes with strong oxidation ability on the valence band of PS I (used to oxidize sacrificial agents) are retained. This characteristic recombination-retention pathway enables both efficient charge separation and strong redox capability. As shown in Figure 5A, the photogenerated electrons in semiconductor B are excited to its conduction band, while holes remain in the valence band. Meanwhile, semiconductor A undergoes a similar excitation process. The key feature is that the electrons in the conduction band of semiconductor A recombine with the holes in the valence band of semiconductor B at the interface (indicated by the downward arrow in Figure 5A), forming a typical “recombination-retention” pathway. Consequently, the electrons with strong reduction ability are preserved in the conduction band of semiconductor B, while the holes with strong oxidation ability remain in the valence band of semiconductor A. Compared to conventional type-II heterojunctions, this architecture offers two distinct advantages. First, it enhances directional charge transfer, wherein built-in electric fields, Fermi level equilibration, and interfacial barrier effects synergistically suppress bulk carrier recombination. Second, it preserves thermodynamic driving forces by circumventing the transfer of carriers to weakly redox-active band edges. The retained high-energy electrons have a sufficiently negative chemical potential, which can efficiently overcome the highest energy barrier of the first electron injection step in the CO₂ reduction reaction, and at the same time provide sufficient energy support for subsequent multi-electron coupling processes, ensuring the continuous progress of the reduction reaction. For further reduction of CO₂, especially the further hydrogenation of *CO intermediates or C-C coupling and other complex steps, the value of the Z-scheme heterojunction is not limited to the basic charge separation, but is reflected in three key dimensions: Firstly, the precise retention of high-energy active electrons can provide the power for the further reduction steps with high energy barriers; Secondly, promoting the rapid cross-interface charge transfer can reduce energy loss during the migration process; Thirdly, maintaining the high availability of electrons at the interface ensures that there is always an adequate electron supply at the reduction sites.

In 2020, Wang et al.^[41] reported that the Cu₂O-Pt/SiC/IrO_x composite system achieved the coupled photocatalytic reduction of CO₂ to formic acid (HCOOH) and oxidation of water to oxygen (O₂) through the direct and indirect Z-scheme connection of the reduction and oxidation half-reactions. As illustrated in Figure 5D, the Cu₂O-Pt/SiC/IrO_x system integrates spatially separated reduction and oxidation units, in which Cu₂O-Pt mainly serves as the CO₂ reduction side, while SiC/IrO_x functions as the water oxidation side. The Z-scheme charge-transfer pathway enables the recombination of low-energy carriers while retaining electrons with strong reduction ability for CO₂-to-HCOOH conversion and holes with strong oxidation ability for water oxidation. The enhanced charge separation and utilization efficiency are further supported by Figure 5E and F. The higher HCOOH and O₂ evolution rates indicate that the photogenerated electrons and holes are effectively consumed in the reduction and oxidation half-reactions, respectively. This balanced production behavior suggests that the Z-scheme configuration promotes efficient charge separation and suppresses the reverse reaction, thereby improving the overall photocatalytic CO₂ reduction performance.

Beyond basic composite designs, the integration of multi-component architectures and interface engineering has further amplified Z-scheme performance. In the ternary ZnFe₂O₄/ZnO/CdS system, the Z-scheme charge transport path was constructed by controlling the interface structure to achieve efficient carrier separation^[42]. The yield of CO₂ to CH₄ was significantly higher than that of single or simple composite systems. The Z-scheme heterojunction constructed through synergy of interface and defect engineering also exhibits remarkable advantages. In the ZnO/ZnAl₂O₄ composite, the introduction of oxygen vacancies and interface defects not only improves the interface charge transfer impedance but also promotes the formation of key intermediates (such as *COOH, *HCO₃^-), making the conversion performance of CO₂ to CO approximately three times that of pure ZnAl₂O₄^[43]. At the same time, both the quantum efficiency and the interface charge separation efficiency have been enhanced. This indicates that the Z-scheme interface has a significant effect in enhancing charge directional migration and reaction kinetics. Recently, some novel Z-scheme heterojunctions have been reported. For instance, in Z-scheme CuS/PCN-222 heterojunction, an intimate contact and efficient electron-hole separation were achieved at the interface between CuS and PCN-222, thereby further enhancing the CO₂ reduction activity and carrier utilization rate. This demonstrated the development potential of Z-scheme MOF/sulfide heterojunctions in PCR^[44]. Such studies generally indicate that the charge dynamics in Z-scheme heterojunctions can be significantly optimized by regulating the charge separation pathways through interface structure design (such as interface chemical coupling, defect introduction, and multiphase energy level platforms). Beyond constructing intimate interfacial contact, the introduction of interfacial chemical bonding has emerged as a more effective strategy for regulating charge transfer. The formation of covalent or strongly coupled chemical bonds at heterojunction interfaces can not only significantly reduce interfacial charge transfer resistance, but also provide stable and continuous electron transport pathways, thereby enabling more efficient charge separation and directional migration. Recent studies further reveal that interfacial chemical bonding plays a crucial role in charge transfer regulation. As illustrated in Figure 5B and C, the introduction of interfacial bonds (e.g., Mo-S bonds) can establish strong electronic coupling, optimize band alignment, and facilitate directional charge migration. For instance, in a Bi₁₉S₂₇Br₃/g-C₃N₄ direct Z-scheme system, the construction of interfacial C-S bonds establishes strong electronic coupling between the two components^[45]. This not only accelerates the transfer of photogenerated electrons from g-C₃N₄ to Bi₁₉S₂₇Br₃, but also increases interfacial electron density and enhances CO₂ adsorption capability. Although the CO₂ adsorption energy of the Bi₁₉S₂₇Br₃/g-C₃N₄ composite is slightly lower than that of pure Bi₁₉S₂₇Br₃, this limitation is effectively compensated by the significantly improved charge separation efficiency arising from the Z-scheme heterojunction and interfacial C-S bonds, ultimately leading to markedly enhanced photocatalytic CO₂ reduction performance^[45]. Charge density difference analysis further confirms that such interfacial bonds act as efficient charge transfer channels [Figure 5F]. Compared to traditional type-II heterojunctions in which electrons/holes separately migrate to lower energy levels and lose their reduction/oxidation capabilities, the Z-scheme heterojunction effectively retains high-energy carriers and reduces bulk-phase recombination through interface advantages^[38], thereby achieving high charge separation efficiency in CO₂ photoreduction reactions and maintaining strong redox driving forces, which is conducive to the complex multi-electron CO₂ reduction (such as the formation of CH₄, C2+ products)^[39].

In addition, Figure 5G summarizes the photocatalytic CO₂ reduction performance of representative Z-scheme heterojunctions by correlating activity and product selectivity for the same material systems. The performance parameters were collected from representative studies reported in Refs.^[46-62] and further reorganized and visualized by the authors to enable a comparative analysis across different Z-scheme heterojunction systems. The left panel compares the production rates of different reduction products, including CO, CH₄, CH₃OH, and HCOOH, while the right panel presents the corresponding selectivity toward carbon-containing products for each catalyst. This paired comparison shows that Z-scheme heterojunctions not only enhance the overall CO₂ conversion activity but also regulate the product distribution. This unequivocally demonstrates that efficient interfacial charge separation, coupled with the thermodynamic retention of high-energy electrons is the critical determinant in steering the complex reaction pathways of multi-electron CO₂ reduction.

In recent years, step-scheme (S-scheme) heterojunctions have rapidly emerged as a focal point in photocatalysis, driven by their unique capacity to regulate interfacial charge transport through a characteristic S-shaped pathway^[15]. Typically composed of two semiconductors with distinct work functions^[54], the S-scheme architecture leverages this work function differential to drive the formation of an intrinsic electric field. As illustrated in Figure 6A, this internal field, coupled with interfacial band bending and electrostatic potential differences, synergistically establishes the foundation for directional charge migration. Mechanistically, an S-scheme system couples an OP with a RP. Upon illumination, the synergistic driving forces compel photogenerated carriers to follow an S-shaped transport route, where low-energy electrons in the conduction band of the OP preferentially recombine with low-energy holes in the valence band of the RP. Consequently, high-energy electrons with robust reducing capabilities (in the RP) and high-energy holes with potent oxidizing capabilities (in the OP) are selectively preserved and directed to their respective active sites. This selective retention mechanism inherently achieves both spatial charge separation and directional charge transfer. Building upon this concept, dual S-scheme heterojunctions, as illustrated in Figure 6B, further introduce multi-interface coupling to construct cascade built-in electric fields, enabling stepwise directional charge transfer across different interfaces. In such systems, the photogenerated carriers undergo hierarchical recombination of low-energy electrons and holes at multiple junctions, while high-energy electrons and holes are progressively separated and enriched at spatially distinct active sites. This multi-level charge regulation not only enhances charge separation efficiency but also maximizes the preservation of strong redox potentials, thereby offering superior performance in driving multi-electron CO₂ reduction reactions. Ultimately, the defining thermodynamic advantage of the S-scheme heterojunction is its ability to fully preserve the original, highly active band edge positions of the constituent semiconductors. This circumvents the energy level compromises typical of conventional heterojunctions, ensuring that the composite maintains sufficient thermodynamic potential to simultaneously drive both CO₂ reduction and complementary oxidation half-reactions.

Recently, Xu et al.^[63] reported S-scheme CeO₂/Bi₂S₃ photocatalysis with palladium-catalyzed carbonylation. The CeO₂/Bi₂S₃ system demonstrated a good yield of CO₂ into CO and a high selectivity of approximately 98%, which highlights the comprehensive advantages of the S-scheme structure in enhancing charge separation and product utilization value. Similarly, Zhang et al.^[64] engineered an interfacial S-scheme heterojunction by anchoring ultrasmall copper phosphosulfide onto 2D g-C₃N₄. Fourier transform infrared (FT-IR) spectroscopy confirmed the formation of interfacial P-N chemical bonds, which act as highly efficient charge transport channels. This synergistic coupling of chemical bonding and S-scheme transport elevated the CO generation rate to eight times that of pristine g-C₃N₄.

Beyond enhancing overall activity, S-scheme heterojunctions demonstrate a remarkable capability to regulate product selectivity in photocatalytic CO₂ reduction. As summarized in Figure 6C, unlike conventional systems where product distribution is primarily dictated by intrinsic material properties, S-scheme architectures fundamentally reshape the reaction kinetics of competing reduction routes.

Because the mechanism selectively preserves high-energy electrons with highly negative potentials, it provides the robust thermodynamic driving force required for complex, multi-electron transfer processes. This ensures a continuous supply of active electrons to drive the formation of deeply reduced species, such as CH₄, CH₃OH, and C2+ hydrocarbons, rather than being limited to two-electron products. Consequently, by finely tuning the band alignment, interfacial electric fields, and catalytic active sites, researchers can effectively modulate the competitive pathways of CO₂ reduction.

The product distributions highlighted in Figure 6C provide a comparative overview of the photocatalytic CO₂ reduction performance of representative S-scheme heterojunctions. The catalytic activity and product selectivity parameters were collected from representative studies reported in Refs.^[65-81] and further extracted, reorganized, and visualized by the authors to facilitate cross-comparison among different material systems. The comparison indicates that S-scheme heterojunctions exhibit diverse hydrocarbon production capabilities, which are closely associated with the synergistic regulation of charge separation, redox potential retention, and surface reaction kinetics. By promoting the accumulation of high-energy electrons and stabilizing key intermediates (e.g., *CO and *CHO), S-scheme heterojunctions can facilitate subsequent hydrogenation and C-C coupling processes. These characteristics highlight the potential of S-scheme architectures as versatile platforms for the selective synthesis of value-added solar fuels.

Schottky heterojunctions are highly effective in enhancing charge separation for photocatalytic CO₂ reduction (PCR). For instance, when a metal contacts an n-type semiconductor [Figure 7A and B]^[82], the metal’s typically higher work function drives spontaneous electron transfer across the interface. To satisfy thermodynamic stability, electrons flow from the semiconductor to the metal until their Fermi levels equilibrate. This interfacial equilibration induces local band bending and establishes a built-in electric field, simultaneously forming a depletion layer within the semiconductor. Consequently, a Schottky barrier emerges at the interface, defined by ΦB = Φ_M - X_SM, where Φ_M represents the work function of the metal and X_SM is the electron affinity of the semiconductor. This barrier fundamentally rectifies charge transport by inhibiting the reverse flow of electrons from the metal back to the semiconductor. As a result, the metal acts as a robust “electron sink”, facilitating targeted electron accumulation on its surface and effectively suppressing deleterious charge recombination.

These enhanced photocatalytic performances can be attributed to the favorable interfacial electronic interactions in Schottky structures, which facilitate electron transfer and promote surface reaction processes during CO₂ reduction. Crucially, the functional utility of Schottky junctions extends beyond mere charge separation. In PCR systems, the metal component frequently serves as the primary catalytic active site and CO₂ adsorption-activation center, dynamically regulating the adsorption configuration and electronic structure of key intermediates. Furthermore, when plasmonic metals (e.g., Au, Ag) are employed, localized surface plasmon resonance (LSPR) dramatically amplifies optical absorption. This synergistic coupling of hot-electron injection and the Schottky barrier effect fundamentally fortifies interfacial charge separation and modulates product selectivity. These kinetic advantages are thoroughly corroborated by electrochemical characterizations. As illustrated in Figure 7C, the enhanced transient photocurrent response and diminished electrochemical impedance radius collectively signify improved charge separation efficiency and accelerated interfacial transfer dynamics, ultimately elevating overall PCR performance.

Beyond charge transfer kinetics, nuanced interfacial electronic structures and corresponding reaction mechanisms can be elucidated via advanced in situ spectroscopy [Figure 7D-F]. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) mapping provide deep insights into the electronic interactions and coordination environments of the metal sites. Complementarily, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) visualizes the dynamic adsorption and photo-activation of CO₂ molecules^[84]. Together, these results substantiate that Schottky metal sites operate dually as efficient electron sinks and active centers for intermediate stabilization.

To further validate the general advantages of Schottky architectures beyond individual case studies, a comprehensive literature comparison was performed [Figure 7G]. The integrated radar plot summarizes the reported photocatalytic CO₂ reduction performance of representative Schottky systems. The integrated radar plot in Figure 7G provides a comparative overview of the reported photocatalytic CO₂ reduction performance of representative Schottky architectures. The catalytic activity and product selectivity parameters were collected from representative studies reported in Refs.^[51,83-100] and subsequently extracted, reorganized, and visualized by the authors to enable comparison among different Schottky-type systems and their corresponding single-component counterparts. The comparison indicates that Schottky architectures frequently exhibit enhanced reported catalytic performance, particularly in terms of carbon-containing product formation and selectivity toward specific reduction products, including CO, CH₄, CH₃OH, and HCOOH. These trends can be attributed to the favorable interfacial electronic interactions in Schottky structures, which facilitate electron transfer and may promote surface reaction processes during CO₂ reduction.

Guided by these mechanistic insights, coupling single atoms or highly dispersed nanometals with semiconductors has emerged as a premier strategy for optimizing charge dynamics. For example, in an Ag single-atom anchored TiO₂ system^[101], macroscopic electron trapping-induced coloration is harnessed to bolster electron storage and migration. The isolated Ag atoms act as dominant active sites to stabilize C₁ intermediates, while adjacent Ti sites facilitate H₂O activation for proton supply, culminating in highly selective CH₄ generation^[101]. Molecular and polymeric photocatalytic platforms analogously benefit from Schottky junction integration to direct charge flow and maximize surface utilization. In benchmark Ag/g-C₃N₄ systems^[102], loading high-work-function metals onto the polymer surface constructs stable Schottky interfaces. This architecture prompts the preferential injection and surface accumulation of photogenerated electrons at the metal sites, severely curtailing bulk-phase recombination and prolonging carrier lifetimes, thereby enhancing the CO production rate significantly beyond that of pristine g-C₃N₄. Moreover, integrating 2D conductive phases, such as MXenes, introduces rapid electron transport channels that augment the interfacial electric field while minimizing contact resistance. For instance, the ZnCr-LDH/Ti₃C₂T_x Schottky system^[85], exhibits exceptional CO yield and selectivity under simulated sunlight, validating the dual role of 2D conductive phases in improving electron extraction and reinforcing the Schottky barrier. Additionally, coupling Schottky junctions with defect engineering further amplifies these kinetic efficiencies. In ultrathin SnS₂ layers enriched with sulfur vacancies, the defect sites dramatically expedite proton and electron supply for the complementary water oxidation half-reaction^[103]. By synergizing the electron utilization efficiency of the Schottky junction with the enhanced oxidation kinetics of the defect sites, this dual-modulation approach bypasses the kinetic bottlenecks associated with half-reaction mismatches, amplifying the CO generation rate to nearly eight times that of the pristine material. Consequently, contemporary research on Schottky heterojunctions for PCR prioritizes three main trajectories: (i) transitioning from metal nanoparticles to single-atom or atomic-level dispersions to achieve highly efficient electron extraction and precisely defined active sites; (ii) integrating 2D conductive phases as electron transport layers to construct stronger built-in electric fields and minimize interfacial resistance; and (iii) synergizing Schottky architectures with defect engineering to mutually optimize barrier heights, charge separation, and half-reaction kinetic matching. Collectively, these advancements establish Schottky heterojunctions as highly tunable interfacial modules for dictating charge and reaction pathways.

To consolidate this mechanistic landscape, the fundamental characteristics of various heterojunction architectures are summarized in Table 1. The core divergence among these designs lies in their driving forces for charge transfer and the consequent impact on redox potential preservation. Conventional type-II heterojunctions rely on standard band alignment, which ensures spatial separation but inevitably sacrifices critical redox energy. In contrast, both Z-scheme and S-scheme systems employ selective recombination mechanisms that preserve high-energy charge carriers necessary for multi-electron CO₂ reduction. Conversely, Schottky heterojunctions are uniquely driven by work function differentials, establishing robust interfacial barriers that enforce directional electron extraction and maximize charge utilization efficiency.

This progressive evolution from simple band-aligned separation toward advanced, thermodynamics-preserving charge regulation highlights the critical necessity of coupling efficient carrier separation with robust redox potential preservation, aligning perfectly with the overarching charge evolution framework conceptualized in Figure 4.

Vacancy defects are one of the most common intrinsic defects in crystalline materials. They refer to the defect formed when lattice nodes lack atoms^[108]. Among them, oxygen vacancies (V_O), sulfur vacancies (V_S), and metal vacancies (V_M) are the most representative. In metal oxides (e.g., TiO₂, CeO₂, In₂O₃, ZnO), V_O are typically generated via the detachment of lattice oxygen atoms, leaving behind undercoordinated metal centers and localized electron-rich microenvironments^{[109, 110]}. These localized states serve a dual function: they act as electron-trapping centers that prolong carrier lifetimes by suppressing non-radiative recombination, while simultaneously serving as prime active sites for the adsorption and activation of CO₂ and critical intermediates (e.g., *CO₂^-, *COOH)^[111]. Advancing beyond empirical observations, recent in situ and theoretical studies provide a deeper mechanistic understanding. For example, V_O in CeO₂ induce the formation of active Ce³⁺ species, which lower the energy barrier for the initial single-electron injection into CO₂ via localized charge redistribution^[112]. Rather than acting merely as static binding sites, these vacancies function as dynamic electron reservoirs that dictate the spatiotemporal distribution of interfacial electrons. However, excessive vacancy concentrations can introduce deep trap states that inadvertently accelerate charge recombination. Consequently, contemporary research has pivoted from indiscriminately maximizing vacancy density toward the precise tuning of vacancy type, trap depth, and spatial distribution to achieve an optimal synergy between charge dynamics and catalytic activity.

Similarly, sulfur vacancies (V_S) are predominantly engineered in 2D transition metal dichalcogenides and sulfides (e.g., MoS₂, WS₂, and SnS₂). The introduction of V_S breaks local crystalline symmetry, exposing previously inert metal centers and substantially amplifying surface reactivity without perturbing the macroscopic band structure^[113]. Recent studies reveal a dual regulatory role for V_S^[114]: they introduce shallow defect states that facilitate electron trapping to extend carrier lifetimes, and they trigger an electronic reconstruction of adjacent metal atoms, which strengthens CO₂ binding affinity and expedites electron injection. Density of states (DOS) analyses provide direct evidence for this V_S-mediated electronic reconstruction. As depicted in Figure 8A-C, the introduction of V_S generates distinct mid-gap localized states (denoted as State 1 and State 2) originating from the rehybridization of metal d and chalcogen p orbitals. These electron-rich mid-gap states effectively accumulate photogenerated electrons, mitigating rapid recombination and providing an energetically favorable pathway for charge transfer to adsorbed CO₂^[113]. Catalytically, these electron-enriched V_S sites promote the back-donation of electrons into the antibonding orbitals of CO₂, accelerating the formation of key intermediates such as *CO₂^- and *COOH. This underscores that sulfur vacancies actively orchestrate interfacial charge transfer and molecular activation rather than functioning as passive electron traps. Consequently, V_S engineering is now a cornerstone design strategy for sulfide-based photocatalytic systems.

In contrast to anion vacancies, metal vacancies (V_M) generally demand higher formation energies, typically requiring harsh annealing or reducing atmospheres, but exert a far more profound impact on the electronic structure and catalytic behavior of the host material^[104]. V_M disrupt the local coordination environment, inducing substantial charge redistribution and lattice polarization that enhance both carrier transport and interfacial kinetics. As illustrated in Figure 8D, high-resolution intensity mapping reveals that structurally unsaturated V_M sites trigger pronounced local lattice distortions. These coordination-unsaturated centers induce spatially confined electronic perturbations, fostering charge localization under reaction conditions. This electronic modulation is further corroborated by electron paramagnetic resonance (EPR) spectroscopy [Figure 8E], where a distinct signal at g = 1.95 directly evidences the formation of paramagnetic defect centers associated with Ni vacancies. The intensity evolution of this signal across different treatment conditions highlights that V_M concentration and electronic activity can be dynamically manipulated via post-etching strategies. Catalytically, these V_M-rich domains critically govern adsorption energetics and intermediate evolution. As summarized in Figure 8F, a defective NiTi-TiO₂ system effectively suppresses the formation of thermodynamically stable yet catalytically inert carbonate species, selectively stabilizing the *COOH intermediate, a widely recognized rate-determining precursor for CO₂-to-methanol conversion^[115]. This pathway reconfiguration stems from the synergy between V_M-induced electronic state redistribution (enhancing electron availability) and localized geometric distortions (optimizing proton accessibility). Crucially, this modulation transcends static adsorption to actively accelerate PCET kinetics. By shortening the effective distance between charge carriers and intermediates, these unsaturated sites lower the activation barriers for sequential hydrogenation steps (*CO₂ → *COOH → *CO → *CHO). Furthermore, from a carrier dynamics perspective, V_M in metal oxides (e.g., WO₃, TiO₂, Bi₂O₃) decrease the effective mass of holes, thereby boosting their mobility and expediting bulk-to-surface carrier migration^[116]. In transition metal oxides like Co₃O₄, V_M promote the formation of high-valence metal species, further driving oxidation kinetics^[117]. Thus, metal vacancies operate as multifunctional catalytic motifs that concurrently optimize charge separation, intermediate stabilization, and reaction pathway selectivity.

Ultimately, the formation and behavior of these vacancy defects are intimately governed by the interplay of material composition, crystal structure, and synthetic conditions (e.g., temperature and atmospheric composition). Thermodynamically, vacancy generation must overcome binding-energy-dependent barriers; kinetically, it is dictated by atomic diffusion during thermal or reductive treatments. Table 2 comprehensively summarizes recent advances in vacancy-engineered photocatalysts for PCR. These benchmarks collectively illustrate that whether leveraging V_O in metal oxides, V_S in sulfides, or more complex V_M and dual-defect architectures (V_M + V_S), vacancy engineering remains a paramount strategy. It not only suppresses deleterious charge recombination but also fundamentally dictates the surface reaction pathways and product selectivity in photocatalytic CO₂ reduction.

Doping defects, introduced via the interstitial or substitutional incorporation of metallic or non-metallic heteroatoms, profoundly reconfigure the electronic band structure, local coordination environments, and charge dynamics of host lattices^[109]. Unlike intrinsic vacancies, doping provides a highly tunable platform to actively dictate the spatiotemporal behavior of photogenerated carriers and optimize interfacial catalytic performance.

Non-metal dopants (e.g., N, C, S, P, B) primarily modulate the valence band architecture, inducing electronic state reconstruction and generating localized impurity states^[106]. Beyond merely broadening the optical absorption spectrum, non-metal doping fundamentally governs interfacial charge transfer. For example, synergistic P-S co-doping transforms polymeric g-C₃N₄ into a narrow-bandgap “black” carbon nitride. This architectural modification not only extends light harvesting into the near-infrared regime but also instigates a redistribution of electron density and local structural polarization, dramatically accelerating CO₂-to-CO conversion by mitigating electron-hole recombination^[135].

Conversely, metal ion doping predominantly engineers shallow energy levels near the conduction band and modulates the Fermi level, steering the directional migration of photogenerated carriers. A quintessential example of metal-doping synergy is the Bi-incorporated In₂O_3-x(OH)_y system. Here, the strategic construction of surface frustrated Lewis pairs (FLPs, cooperatively regulated by hydroxyl groups and oxygen vacancies) is further optimized by Bi³⁺ substitution at In³⁺ sites. This targeted metal incorporation finely tunes the surface active motifs, dictating both the thermodynamic activation of CO₂ and the ultimate product selectivity^[136,137].

Extending these principles, dual-doping strategies (e.g., metal/non-metal combinations or dopant-vacancy couplings) elicit robust synergistic effects. For instance, P-K co-doping in g-C₃N₄ amplifies charge separation while driving the multielectron reduction of CO₂ toward CH₄^[138], whereas N-metal co-doping in TiO₂ concurrently broadens spectral responsiveness and enhances product selectivity^[139]. Table 3 systematically catalogs recent benchmarks in doping-engineered photocatalysts, highlighting the macroscopic performance gains achieved through these elemental substitutions.

To bridge these macroscopic catalytic enhancements with atomic-scale charge dynamics, interstitial carbon-doped SnS₂ (SnS₂-C) serves as an elegant mechanistic model [Figure 9A-C]. Interstitial carbon incorporation disrupts local structural symmetry, inducing pronounced lattice microstrain and charge polarization. High-resolution XPS [Figure 9A and B] unveils a distinct positive shift in the binding energies of both Sn 3d and S 2p core levels. This phenomenon signifies partial electron depletion from the host lattice, indicating that interstitial carbon acts as a potent electron-withdrawing locus. Such localized electron redistribution generates a robust built-in electric field that drives the spatial separation of bulk electron-hole pairs, funneling them directionally toward the catalytic interface. Furthermore, as depicted in Figure 9C, the synergy between carbon-induced lattice strain and electronic modulation broadens visible-light harvesting while establishing energetically favorable pathways for charge injection into adsorbed CO₂. Ultimately, this paradigm underscores that doping defect engineering transcends simple bandgap narrowing; it acts as a comprehensive tool to construct internal electric fields, optimize charge routing, and fundamentally dictate the fate of photogenerated carriers at the atomic level.